1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
2. Description of the Related Art
The history of developing semiconductor devices having transistors using semiconductors has been a history of the challenge of providing higher performance semiconductor devices at lower cost.
One of methods for improving the performance of semiconductor devices is to improve crystallinity of a semiconductor.
Single-crystal semiconductors have the highest crystallinity.
Single-crystal semiconductor wafers, SOI (silicon on insulator) substrates, and the like are given as substrates for single-crystal semiconductors.
However, such substrates have a disadvantage of high cost. This disadvantage is more significant as the substrates have larger area.
On the other hand, semiconductor devices using semiconductors formed on low-cost substrates (e.g., glass substrates) by a film formation method are given as low-cost semiconductor devices in which larger substrates are suitably used.
Further, regarding a semiconductor device using a semiconductor formed on a substrate by a film formation method, there have been attempts to provide higher performance semiconductor devices at low cost, in which crystallinity of a semiconductor is improved (for example, Reference 1: Japanese Published Patent Application No. H7-130652).
However, it is very difficult to make a perfect single-crystal semiconductor from a semiconductor formed on a substrate by a film formation method.
In response, there have also been attempts to form a single-crystal semiconductor on a low-cost substrate by bonding a single-crystal semiconductor onto a low-cost substrate (Reference 2: Japanese Published Patent Application No. H11-163363).
However, the techniques disclosed in Patent Document 1 and Patent Document 2 have a problem of damaging a single crystal due to implantation of ion species of hydrogen or the like and a problem of generating large projections and depressions on a surface of a single crystal which remains over a base substrate.
When a gate insulating film is formed, the large projections and depressions penetrate the gate insulating film, causing a problem of leakage between the semiconductor layer and a gate electrode.
With the invention disclosed in Reference 2, high performance semiconductor devices can be provided at low cost; however, since a single-crystal semiconductor is used, the cost is higher compared with the cost involved in the case of using the invention disclosed in Reference 1.
Here, a semiconductor device has various circuits, and there are cases where not all the circuits are necessarily formed using a single crystal, but only some high performance circuits should be formed using a single crystal.